High power tolerant (10-20 W) LC filter

Question

I need to build a passive high-pass filter that has a cutoff frequency of 2-3 kHz. Sounds simple? It needs to pass between 10 and 20 W rms power at 20 kHz without overheating and going "bang".

The purpose of this circuit is to supply AC power to a nichrome wire heating element avoiding n*50 Hz amplifier noise due to mains electricity frequency. The rms voltage is below 20 V and the current is below 1 A.

In a previous attempt, I tried to build the circuit below, where R1 is the nichrome wire heater and C1 is two opposite 100 uF electrolytic capacitors in series (cathodes joined together):

This worked for about 10 seconds, then one of the capacitors exploded:

My questions are:

• What do you think went wrong with my attempt? e.g. Was a 35 V capacitor rating not high enough, or was cooling needed?

• What would you suggest will give the circuit the best chance of working as intended.

Answer 1

TL/DR: use a big film cap, like a motor cap.

DC also creates magnetic background that I want to avoid, so that method is ruled out.

Use a non-inductive pattern for your nichrome wire, this will cancel the magnetic field out.

What do you think went wrong with my attempt? e.g. Was a 35 V capacitor rating not high enough, or was cooling needed?

A 100µF general purpose cap has about 1-2 ohms ESR, which is resistance so it will convert current into heat. This creates enough heat to explode the capacitor. It is quite small, it can't dissipate too much heat.

You need a low-ESR cap. A good idea would be a cap specified for a ripple current higher than the AC current you run through it. (that's the idea with the ripple current spec).

Ceramic has voltage-dependent capacitance, but the voltage across the cap will be small. So, ceramic may be an option. You can use any parallel combination of caps optimized for lower cost, like 10x 10µF 25V caps. Be aware that it will add a bit of distortion on the waveform.

It will be tricky to find 100µF aluminium caps with low enough ESR and high enough ripple current, but you can use polymer caps, which should work well.

Answer 2

This is the proper way to build a non-polarized capacitor out of ordinary electrolytics. The diodes are mandatory to protect each capacitor from reverse polarity. That is possibly what blew them before. Now the most reverse voltage across each one will be about -0.7 volts.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

Your filter:                My Filter:

Type: 2nd order HPF         3rd order HPF
Peak: 8 kHz +12 dB          +13 dB @20 kHz
Attenuation:   
   100 Hz:  -75 dB          -130 dB
  1000 Hz:  -35 dB           -70 dB

^{simulate this circuit – Schematic created using CircuitLab}

ECWF4754JL  0.75 uF film PP radial ( 3pc ) 

The paired parts carry more than twice the current here at resonance thus shared current.

If you wind your own magnet wire choke, you can drive > 100 W rms or >200W peak in 4 Ohms.
30uH into 34 Ohms,
40uH into 4 Ohms for 20kHz

Why under-design for 10~20W when you can over design for 100W?